The Montara oil rig as it leaks fuel into the Timor Sea, September 2009. Photograph: Annabelle Sandes/Kimberley Whale Watching/WWF

Conservationists warned yesterday that one of Australia's worst off-shore oil spills was killing wildlife and "massively contaminating" one of the world's last great wildnernesses. Amid a fourth attempt to plug the 64-day-old leak at the Montara drilling rig, the slick – which has already spread over an area 10 times the size of London – continued to expand at the rate of 300 barrels of oil a day in an area of the Timor Sea famed for its marine reserves and coral.

A survey by the Worldwide Fund for Nature found dolphins, migratory sea birds, sea snakes and marine turtles were exposed to toxins. The slick has killed hundreds, possibly thousands, of animals.

Since August 21 when there was an accident at the Montara offshore drilling rig's well head, around 403,000 litres of oil have been pumped into the Timor Sea. The rig is owned by the Thai oil company PTTEP.

Satellite images show a 25,000 square kilometre slick spreading across the surface of the ocean and spilling into Indonesian waters, threatening the marine reserves of Ashmore and Cartier reefs along the way.

WWF director of conservation Dr Gilly Llewellyn, who conducted a three day expedition through the polluted waters, said if the spill were closer to shore there would be global outrage. "There were times when we were literally in a sea of oil from left to right and as far as we could see ahead of us - it was heavily oiled water and it was sickening because in this we were seeing dolphins surfacing," Llewellyn said.

"We recorded hundreds of dolphins and sea birds in the oil slick area, as well as sea snakes and threatened hawksbill and flatback turtles. Clearly, wildlife is dying and hundreds if not thousands of dolphins, seabirds and sea-snakes are being exposed to toxic oil," Llewellyn said.

The expedition recorded 17 species of seabird, four species of dolphins and five marine reptiles including two species of marine turtle.

However the paucity of research on the marine life in the area has hampered attempts to document the damage.

Still, Llewellyn says experience from previous oil disasters suggests the damage will be long lasting. "We know that oil can be a slow and silent killer. Impacts from the Exxon Valdez disaster are still being seen 20 years later, so we can expect this environmental disaster will continue to unfold for years to come," she said.

When oil, gas and condensate began seeping into the Timor Sea PTTEP estimated it would take 50 days to plug the well which is located about 250 kilometres off the Kimberley region of the West Australian coast line. Three previous attempts by PTTEP Australasia to plug the leak, 2.5 kilometres below the sea bed by pumping it full of heavy mud have failed.

Australia's federal environment minister, Peter Garrett, said he was confident everything possible was being done to stop the oil leak.